My experience the past two years of these types of tests has been mixed. Some students consent to taking the test with no problem; others moan, groan, and refuse unless you make a deal with them. Classes are supposed to be completely silent and over the past two years it has been quite hectic. I truly believe it is not the teacher's fault in many of these scenarios - especially when you have 30 students in a classroom and you need to make sure they all have the correct supplies out and their cell phones put away (And you wonder why neighborhood schools score lower?).
In my classroom the vast majority of students finished the test with time to spare and when I asked them to rearrange the desks back into small table-groups it took about 45 seconds for them to complete the task. I have asked similar things in years past and been met with reactions like, "why should I do that - it's not my class!"
While there are some problems in magnet school testing situations (especially amongst the older grades) it seems a lot easier than neighborhood school situations.